Technical Field
Field of the Invention is wireless telephony using the IP Multimedia Subsystem (IMS) which applies the Real-time Transport Protocol (RTP) to carry Voice over IP (VoIP). A refinement, Voice over Long Term Evolution (VoLTE), utilizes high data rate cellular networks for full duplex voice calls with improved efficiency.
The class of the subject matter is within telecommunications and more specifically in diagnostic testing and measurement for evaluating the end-to-end condition of a cellular radiotelephone system having a transmitter and receiver at separate stations.
Description of the Related Art
Real-Time Transport Protocol
With Voice Over IP calls, voice is encoded and transmitted in RTP packets. If the transmission time of packets from one end to another is too long, the delay in hearing the other party makes users ‘talk’ over each other.
The RTP RFC-3550 describes the interarrival jitter which conventionally evaluates the quality of the voice call (if the jitter is too high, not enough packets arrive on time for the call quality to be good) but as it is computed for each end of the call, it does not measure the conversational latency or delay.
Conventionally, it is possible to evaluate the delay in a laboratory setting with test equipment but not possible to know the delay for actual customers of the mobile operator.
The generally accepted limit for good quality voice connection delay is 200 ms one-way or 250 ms as a limit. As delays rise over this threshold, talkers and listeners become uncomfortable, un-synchronized, and often they speak at the same time or both listen for the other to speak. This behavior is described as talker overlap. While the overall voice quality is acceptable, users subjectively evaluate the stilted nature of the conversation unacceptably annoying. Talker overlap is easily appreciated on international telephone calls when they travel over geosynchronous satellite connections because at the speed of light, satellite delay is in the order of 250 ms up and 250 ms down.
Conventional systems depend upon subjective user reported incidents and descriptions or objective networking statistics of packet loss and jitter. But combining these into a mean opinion score (MOS) leaves a gap for some aspects of user perception. One such tangible user perception is mouth-to-ear delay, also known as conversational latency, or duplex delay.
What is needed is a way to better measure the Quality of Service and Mean Opinion Score experienced by smartphone consumers. Reliable measurement of conversational latency or duplex delay in each direction would help adoption.
The unmet need is to evaluate the average end-to-end delay in transmitting voice, for calls performed on a live network. Mobile phone operators could detect issues with delay for IMS calls (even if they are not reported by users) and then investigate and fix the cause of the issues if this key performance indicator could be accurately estimated based on real data.